The present invention generally relates to a vehicle steering system and, more particularly, relates to a steer-by-wire steering system in which there is no direct mechanical linkage between the hand-operated input (e.g., steering wheel) and the road wheels.
Steering of a wheeled motor vehicle is typically achieved by a vehicle operator (i.e., driver) rotating a hand-operated steering wheel that is arranged in the passenger compartment of the vehicle to command turning of the steerable road wheels. Conventional steering systems generally include a rack and pinion-type steering assembly operatively coupled to the steerable road wheels and a steering column coupled between the steering wheel and the rack and pinion assembly for converting angular rotational movement of the steering wheel into sliding movement on a linear actuator to effect turning of the road wheels. In the past, many vehicle steering systems commonly employed a direct mechanical linkage connected between the steering wheel and the rack and pinion assembly. When the steering wheel is rotated, the mechanical linkage transfers the rotational force to the rack and pinion assembly which, in turn, actuates the steerable road wheels to be turned left or right, typically with the assistance of hydraulic-powered or electric-powered assistance.
A more advanced steering system which has no direct mechanical linkage between the steering wheel and the steered road wheels is commonly referred to as a steer-by-wire steering system. Steer-by-wire steering systems that have been proposed and/or employed in automotive vehicles typically include a steering column having a rotatable steering wheel shaft mechanically connected to the driver actuated steering wheel and extending into a housing in the steering column. A feedback motor and an angular position sensor are coupled to the steering wheel shaft. The steer-by-wire steering system also employs a pinion shaft which is not mechanically linked to the steering wheel shaft. Instead, the pinion shaft is rotary driven by an actuator, such as an electric motor, typically in response to the sensed angular position of the steering wheel shaft, to actuate the rack and pinion assembly so as to turn the road wheels. In addition, a torque sensor is typically also positioned on the pinion shaft to sense torque which, in turn, is used to control the amount of feedback force applied to the steering wheel via the feedback motor.
Mechanically linked steering systems generally have road wheel travel turning limits which limit the rotational range of motion of the steering wheel. Typically, these rotary limits are imposed by the travel limits of the rack and pinion assembly which, in turn, prevents further rotation of the steering wheel due to the interconnecting mechanical linkage. In a steer-by-wire steering system, since there is no mechanical linkage between the steering wheel and the rack and pinion assembly, the rack and pinion assembly does not limit the rotational travel of the steering wheel. Thus, in some steer-by-wire steering systems the steering wheel may be rotated without rotational limitation. While it is conceivable that the feedback electric motor may be used to resist travel at predefined rotational limits, the use of the feedback motor to resist rotational movement would typically draw high current and thus consume electric power. Further, when electric power is shut off to the motor, no such steering wheel travel limits would be imposed by the motor.
Accordingly, it is desired to provide for a steer-by-wire steering system for a vehicle that limits the rotational travel of the driver commanded steering wheel.